Systems, methods and devices for dual closed loop modulation controller for nonlinear RF amplifier

ABSTRACT

In accordance with various exemplary embodiments of the present invention, systems, methods and devices are configured to facilitate RF envelope amplitude control. For example, a RF envelope amplitude control system comprises: a RF amplifier, wherein the RF amplifier is associated with a feedback device that is configured to create a first feedback signal representing the power in an RF output signal; a transmit waveform generator configured to generate a reference waveform signal; an adaptive table waveform generator configured to compare the reference waveform signal and the first feedback signal and to create a second feedback signal based on that comparison; and a loop filter configured to combine the reference waveform signal, the first feedback signal, and the second feedback signal to form an amplifier control signal, wherein the amplifier control signal is provided to the RF amplifier to adjust the RF output signal to conform to a specified RF envelope.

FIELD OF INVENTION

The present invention generally relates to RF amplifiers, and moreparticularly, to envelope controllers for nonlinear RF amplifiers.

BACKGROUND OF THE INVENTION

In some types of radio frequency (“RF”) transmitters, the RF outputsignal of an RF amplifier is required to closely match one of severalpredetermined amplitude envelopes. For example, RF transmitters existwhere the transmitter output consists of continuous wave (“CW”) or phasemodulated RF pulses that are required to closely match one of severalpredetermined amplitude envelopes. One such transmit mode is known asTactical Air Command And Navigation (“TACAN”) involving amplitudemodulation. Another is Time Domain Multiple Access (“TDMA”) involvingphase modulation. Unfortunately, RF transmitters and their componentsare often subject to relatively severe temperature stresses, which canmake it more difficult to match the desired waveform envelope.

In addition, transmitter frequency range, output power, size andefficiency requirements associated with, for example TACAN/TDMA, havedictated the use of highly nonlinear (typically ‘class C’) devices inthe RF power amplifiers. To meet the envelope accuracy requirements ofthese nonlinear devices, closed loop control has been used to reduce thedifference between the actual RF amplifier output waveform and a desired(as set by industry standards) waveform. However, during the intervalbetween transmitted pulses, no RF energy is allowed to be transmitted;therefore a closed loop condition cannot exist during this interval.

A single control loop, operating under this constraint, havingsufficient gain and bandwidth to meet the accuracy requirements oversome portions of the output envelope, will likely be unstable over otherportions of the output envelope. The instability is due primarily to thenonlinearities in the controlled element (the actual RF amplifier chain)and the absence of a valid closed loop condition in the time intervalbetween RF pulses.

The control of transmitted RF pulses by single, analog closed loops iswell known (e.g., Philips part #PCF5708, “Power Amplifier Controller forGSM and PCN Systems”, the data sheet for an IC controller). However, thewaveform rise and fall times for GSM are approximately 30 μsec versus3.5 μsec for Joint Tactical Radio System (“JTRS”) and TACAN and 0.25μsec for TDMA, therefore the previous solutions do not provide therequired control bandwidth.

Existing controllers have also generally been used with RF amplifiersthat exhibit an approximately linear control transfer function (e.g.,gate voltage control in a GaAs FET module). By contrast, due to the lowgain of the devices required for JTRS-compatible applications, manystages are needed to achieve the required gain and output power. To meetefficiency requirements, each stage is normally operated under ‘class B’or ‘class C’ conditions, and the RF input to RF output amplitudetransfer function of the cascaded or series connection of multiplestages is therefore highly nonlinear.

Conventionally, the closed loop output distortion (the deviation of theRF output envelope from an ideal shape) is minimized by increasing boththe closed loop gain and bandwidth up to the stability limit imposed byhardware delays and phase shifts within the closed loop. By contrast,the maximum allowed spurious, or modulation dependent, RF sidebandoutput levels of a JTRS/TACAN transmitter are relatively low, and can beseriously degraded (increased) by excessive gain in the waveform controlloop. In practice, therefore, the selection of the optimum loop gain fora given set of conditions is a compromise between the conflictingrequirements of waveform distortion and minimum sideband noise. Thisoptimum point varies with temperature, required power level andoperating frequency.

Thus, there exists a need for an RF envelope amplitude controller thatis configured to achieve RF output envelope control in applicationshaving high frequencies, and/or nonlinearities, such as TACAN/TDMAapplications. There also exists a need for a flexible RF envelopeamplitude controller that can readily adjust for temperature, operatingpower mode, operating frequency, and waveform changes.

SUMMARY OF THE INVENTION

In accordance with various exemplary embodiments of the presentinvention, a RF envelope amplitude control system comprises: a RFamplifier configured to receive a RF input signal and to output a RFoutput signal, wherein the RF amplifier is associated with a feedbackdevice that is configured to create a first feedback signal representingthe power in the RF output signal; a transmit waveform generatorconfigured to generate a reference waveform signal; an adaptive tablewaveform generator configured to compare the reference waveform signaland the first feedback signal and to create a second feedback signalbased on the difference between the reference waveform signal and thefirst feedback signal; and a loop filter configured to combine thereference waveform signal, the first feedback signal, and the secondfeedback signal to form an amplifier control signal, wherein theamplifier control signal is provided to the RF amplifier to adjust theRF output signal with respect to a RF envelope.

In another exemplary embodiment, a transmitter system comprises: an RFamplifier; and an RF envelope amplitude controller; wherein the RFenvelope controller is configured to provide an amplifier control signalto the RF amplifier and to receive a first feedback signal from the RFamplifier, wherein the amplifier control signal is partially based on areference waveform signal; wherein the first feedback signal representsthe power of an RF output signal from the RF amplifier, wherein the RFenvelope amplitude controller is configured to modify the amplifiercontrol signal by the first feedback signal; wherein the RF envelopeamplitude controller is further configured to compare the first feedbacksignal to the reference waveform signal and to create a second feedbacksignal based on the comparison; and wherein the RF envelope amplitudecontroller is configured to further modify the amplifier control signalbased on the second feedback signal.

In another exemplary embodiment, a RF envelope amplitude control methodcomprises the steps of: receiving a reference waveform signal at an RFenvelope amplitude controller; receiving a feedback signal, based on theRF output of an RF transmit amplifier, at the RF envelope amplitudecontroller; comparing the feedback signal to the reference waveformsignal; adjusting an adaptive table to store a waveform value at amemory location in the adaptive table; combining the reference waveformsignal, the first feedback signal and the second feedback signal to forman amplifier control signal that facilitates adjusting the amplifier RFoutput signal to achieve a close approximation of a desired waveformenvelope.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the drawing Figures, wherein like reference numbersrefer to similar elements throughout the drawing Figures, and:

FIG. 1 is a block diagram overview of an exemplary transmitter system;

FIG. 2 is a block diagram of another exemplary transmitter system;

FIG. 3 is a block diagram of yet another exemplary transmitter system;

FIG. 4 illustrates the DAC output voltage at clock cycles in anexemplary transmitter device;

FIG. 5 is a block diagram of an exemplary conversion component in anexemplary embodiment of the present invention;

FIGS. 6 & 7 illustrate exemplary timing diagrams; and

FIG. 8 is a flow diagram showing an exemplary method of controlling RFenvelope amplitude.

DETAILED DESCRIPTION

While the exemplary embodiments herein are described in sufficientdetail to enable those skilled in the art to practice the invention, itshould be understood that other embodiments may be realized and thatlogical and mechanical changes may be made without departing from thespirit and scope of the invention. Thus, the following detaileddescription is presented for purposes of illustration only and not oflimitation.

In accordance with various exemplary embodiments of the invention,systems, methods and devices are configured for providing RF envelopeamplitude control. The control system is configured to facilitateamplification, in non-linear RF amplifiers, of RF signals prior totransmission. Moreover, the control system is configured to facilitateamplification of RF signals having stringent requirements on thewaveform output envelopes.

In accordance with an exemplary embodiment of the present invention, atransmitter system is configured to receive a transmit RF signal,amplify that signal or otherwise process that signal, and output anamplified/processed signal that substantially conforms to specificwaveform output envelope requirements. Furthermore, the transmittersystem may be used in connection with higher level assemblies. In oneexemplary embodiment, the transmitter system is configured for use inairplane avionics, such as in Multipurpose Information DistributionSystem (“MIDS”)—JTRS. Although described herein in terms of atransmitter RF amplification system, the system and method is equallyapplicable to any non-linear processes. For example, the transmittersystem may be configured for use in one or more individual nodes of anRF repeater network, or for use in one or more individual nodes of anoptical repeater network.

In accordance with various exemplary embodiments of the presentinvention, the transmitter system comprises an RF envelope amplitudecontroller used in a RF transmitter for a specific application. Theoutput of the transmitter consists of RF pulses that are amplitudeand/or phase modulated. By way of example, the transmitter RF outputvoltage pulses are described herein to consist of two different,standard shapes (hereafter termed ‘waveforms’ or ‘envelopes’), commonlydesignated TACAN and TDMA. However, other waveforms may also be used invarious exemplary embodiments of the present invention.

In accordance with various exemplary embodiments of the presentinvention, a transmitter system is configured to overcome theinstability experienced in prior art solutions through the use of twoclosed control loops. The two closed control loops exploit a performanceadvantage based on the complete a priori knowledge of the relevantrequired transmitted pulse amplitude envelopes. A first closed loopcontributes a repetitive or stationary component of the solution to therequired modulator input. A second loop acts to reduce thenon-stationary, or residual, portion of the error between theapproximately corrected RF output and the desired waveform. Comparedwith the gain required in a single loop solution, substantially lessgain is required of the second loop, thereby improving stability.

In accordance with one aspect of an exemplary embodiment, thetransmitter pulse shaping is realized by amplitude modulation, utilizingtwo modulation controller functions. The desired shaping or modulationis applied to an externally supplied RF input pulse, which issubstantially a rectangular pulse, i.e., the peak sinusoidal carriervoltage is essentially constant during the input pulse. A closed loopcontroller ‘B’ is then used to reduce and/or minimize the error betweenthe measured RF amplifier output voltage waveform and a referencewaveform. The closed loop bandwidth of the ‘B’ controller is configuredto substantially correct intra-pulse amplitude distortion introduced bynonlinearities in the amplifier stages of the transmitter.

During the interval between transmitted pulses, no RF energy is allowedto be transmitted; therefore the ‘B’ loop is not closed during thisinterval. Additionally, according to well known principles, the RF inputlevel to an active device operating in “class C” mode must exceed aminimum threshold or level before useable energy appears at the deviceoutput. This threshold is heavily dependent on external factors such asoperating frequency and ambient temperature. It is therefore highlydesirable that, immediately prior to the start of an RF input pulse, themodulation control point exists in a state that approximately effectsthe correct ‘class C’ device RF threshold voltage, despite the absenceof meaningful input to the ‘B’ controller.

In accordance with various exemplary embodiments of the presentinvention, the transmitter system includes a closed loop controller ‘A’comprising a waveform generator and digital memory. The error inputsignal (feedback signal) is common to both the ‘A’ and ‘B’ controlloops. The primary purpose of controller ‘A’ is to generate a correctiontable (or waveform), synchronized in time with the reference waveformfor the ‘B’ loop.

Unlike the reference waveform, the contents of the correction table canadapt, or change, over multiple pulses, based on the common loop errorsignal. In one exemplary embodiment, this error is evaluated at everydata point (memory location) of the reference waveform table. However,in other embodiments, the error may be evaluated at other intervals,e.g., at every second or third data point. In an exemplary embodiment,if the error magnitude exceeds a fixed threshold, the contents of thecorrection (adaptive) table are incremented or decremented by a fixedinteger value. The resulting adaptive table waveform may be summed withthe reference waveform and input to the loop filter.

In accordance with various aspects of an exemplary embodiment, the ‘A’loop adaptive table waveform is active (i.e., converges to non zerovalues) primarily at the beginning and the end of the desired RF pulse,when the gain of the ‘B’ loop is undefined due to the sudden presence orabsence of the loop error signal. The control loop ‘A’ bandwidth is muchlower than that of the ‘B’ loop, and its output provides anapproximately correct value of the modulation control signal input tothe transmitter. The closed loop gain requirement for the ‘B’ loop isthereby reduced.

Transmitter System 10:

In accordance with one aspect of an exemplary embodiment of the presentinvention, and with reference to FIG. 1, an exemplary transmitter system10 comprises an RF amplifier 40 and an RF envelope amplitude controller60. Transmitter system 10 may further comprise a system controller 90.

In accordance with an exemplary embodiment, amplifier 40 is configuredto receive an RF input signal, amplify and/or amplitude modulate the RFsignal, output a RF output signal, and provide a sample of the RF outputsignal. In this regard, it explicitly stated that the amplification maybe made by any suitable amplifier. In one exemplary embodiment of thepresent invention, amplifier 40 is a class C amplifier. In variousexemplary embodiments, amplifier 40 is a non-linear amplifier. However,it should be noted that although described herein as an RF amplifier,amplifier 40 may be any non-linear device, including optical processingfunctional blocks, such as fiber optic transmitters or repeaters.

Amplifier 40 is further configured to receive a control signal thatcontrols the RF output power of the amplifier. In addition, amplifier 40is configured to provide a feedback signal based on the RF outputsignal.

In accordance with an exemplary embodiment, amplifier 40 is a Part No.1036159 by US Monolithics. However, amplifier 40 may comprise anysuitable non-linear amplifier that is configured to receive an amplifieroutput power control signal and that is configured to generate afeedback signal.

In accordance with an exemplary embodiment, RF envelope amplitudecontroller 60 is configured to receive a system controller signal. Inone exemplary embodiment, the system controller signal comprises a clocksignal and a control signal. Furthermore, the system controller signalmay comprise other signals. The signal may be received from, forexample, system controller 90. System controller 90 may be a singledevice or multiple devices configured to provide the signals describedherein.

In accordance with various exemplary embodiments, system controller 90is configured to provide a control signal that includes informationrelated to the desired waveform of the RF output signal of theamplifier. For example, the control signal may be a memory address thatis related to a waveform stored therein. In another exemplary embodimentof the present invention, the control signal is a reference waveformsignal. In one exemplary embodiment, the reference waveform is a firstapproximation of the amplifier control signal that will cause amplifier40 to generate an RF output signal approximating the desired waveformenvelope. In yet another exemplary embodiment, the control signal is acode that facilitates “looking up” the reference waveform signal.

In a further exemplary embodiment, the desired waveform (or referencewaveform) is represented by an integral number M of predefined amplitudevalues that are stored digitally. After receiving an external command tobegin an RF output power control sequence, M waveform time-samples arethen sequentially provided to the RF envelope amplitude controller 60 ata rate determined by a system clock. After the Mth time-sample iscommunicated to the amplitude controller, the RF output control sequenceis terminated, and the amplitude controller is placed in a condition tocorrectly respond to another external command to begin another RF outputpower control sequence. In this exemplary embodiment, system controller90 is configured to provide all of the waveform time-sample counting,arithmetic, and control functions that are minimally required for thecorrect operation of the RF envelope amplitude controller.

In one example, the control signal may indicate that the desiredwaveform is such as would be suitable for TACAN, TDMA, Frequency ShiftKeying (“FSK”), Gaussian Minimum Shift Keying (“GMSK”), Minimum ShiftKeying (“MSK”), Phase Shift Keying (“PSK”), or any other waveformwherein (a) communication or navigation utilizes RF or optical pulses,(b) all pulses within any message exhibit approximately the same RF oroptical power envelope, and (c) the envelopes of all pulses within anymessage are approximately independent of message content. Furthermore,it is contemplated that this control signal may communicate any known orfuture developed waveform or reference thereto. This control signal mayalso communicate an on/off mode to the RF envelope amplitude controller60.

RF envelope amplitude controller 60 is further configured to provide anamplifier control signal to amplifier 40 that is at least in part basedon the system controller signal and that reduces the difference betweenthe conversion component output signal and the reference waveform.Stated another way, RF envelope amplitude controller 60 is configured tocommunicate an amplifier control signal that reduces the differencebetween the amplifier RF output signal and a desired (industry standard)waveform/envelope.

RF envelope amplitude controller 60 also is configured to receive afeedback signal from amplifier 40, wherein the feedback signal is basedon the RF output signal. For example, the feedback signal may be arepresentation of the actual waveform, or any suitable signal that canbe interpreted to facilitate making adjustments to the amplifier controlsignal to cause the RF output signal to more closely exhibit the desiredoutput waveform.

In accordance with an aspect of an exemplary embodiment of theinvention, RF envelope amplitude controller 60 is configured to adjustthe amplifier control signal that it provides to amplifier 40 based ondual feedback loops. This adjustment is based on a first feedback signalfrom the amplifier (loop 1), and is further based on a second feedbacksignal (loop 2) based on a comparison of a reference waveform signal tothe first feedback signal.

In this exemplary embodiment, the first feedback loop adjusts theamplifier control signal in order to reduce and/or minimize the shortterm (i.e., over the duration of individual pulses) error between thereference waveform and the first feedback signal. The second feedbacksignal adjusts the amplifier control signal in order to reduce thelong-term (i.e., based on many pulses) or averaged error between thereference waveform and the first feedback signal.

Due to the close similarity of adjacent pulses for the applicableformats (e.g., TACAN, TDMA, FSK, GMSK, MSK, PSK), the second looptherefore also reduces the error between any individual amplifier outputpulse and the desired waveform. In general, the second loop isconfigured to provide a predictive and approximate waveform feedbacksignal for the purpose of achieving an approximation of the appropriatecorrection to the amplifier control signal. Although this predictivewaveform is adjusted from time to time to account for changes intransmission mode, changes in temperature, or the like, it should berecognized that the second loop is not configured to be highly accurate.Rather, at least one purpose of the second loop is to reduce the amountof correction required to be supplied by the first control loop.

Thus, the gain requirement for the first loop is reduced. For a givengain-bandwidth product requirement imposed upon the first loop, areduction in the required first loop gain may result directly in agreater available bandwidth within the first loop. Phase shifts withinthe first loop are thereby minimized, resulting in a relativeimprovement in loop stability and response time.

In addition, transmitter system 10 may be installed in higher levelassemblies such as RF or optical messaging transmitters or repeaters,for example. In some embodiments, the higher level assemblies maycomprise an antenna 41 configured to transmit signals to a remotelocation. The output of amplifier 40 may be communicated directly toantenna 41, or may first be processed by other (typically linear)components before being communicated to antenna 41.

In accordance with one exemplary embodiment, amplifier 40 is describedin further detail with reference to FIG. 3, in which the transmittersystem comprises an amplifier 400. In accordance with one exemplaryembodiment of the present invention, amplifier 400 comprises RF input403, and RF output 440, amplifier control signal input 404, and feedbackoutput 503. Amplifier 400 may further comprise RF amplifiers 425 and430, double balanced, diode ring mixers 415 and 420, diodes 405 and 410,resistances 407 and 412, and RF output feedback device 435.

In this exemplary embodiment, diode ring mixers 415 and 420 areconfigured to function as high speed current controlled RF attenuators.Series resistances 407 and 412 are configured to cause the controlcurrents passing through diodes 405 and 410 to be approximately equalwhen driven over the normal range (e.g., zero to +2 V) of the amplifiercontrol voltage signal provided at amplifier control signal input 404.Diodes 405 and 410 are configured to prevent the flow of current ofincorrect polarity. Buffer preamplifier 425 is configured to provide anacceptable system interface impedance match at RF input port 403 despitevariations in the terminal impedance of mixer 415 as a function ofcontrol current. Mixers 415 and 420, and amplifiers 425 and 430 are, forexample, configured to provide a gain that is high enough to produce themaximum available power from amplifier 430 when the control signal 404is at its maximum design value. The maximum design value of the controlcurrent is that current higher than which no substantial decrease in theattenuation of both 415 and 430 is obtained.

Feedback device 435, in one exemplary embodiment of the presentinvention, is configured to provide a sample of the RF output signal ofamplifier 400 that facilitates adjusting the amplifier control signal atinput 404 to achieve a desired adjustment to the RF output signal at RFoutput 440. In accordance with one aspect of an exemplary embodiment ofthe present invention, feedback device 435 is configured to provide asignal that reflects the RF power level of the RF output signal. Forexample, feedback device 435 may be a passive RF directional coupler. Inanother exemplary embodiment, feedback device 435 is a non-directionalcoupler.

Moreover, feedback device 435 may be part of amplifier 400 or may be aseparate component used in conjunction with amplifier 400. Thus,feedback device 435 may be any device(s) configured to provide a signalbased on and/or indicative of the RF power output of amplifier 400.

It will be apparent that various different components, and amplifierdesigns may be used to create an amplifier, and the amplifier describedwith reference to FIG. 3 is only one exemplary amplifier. Thus, inaccordance with various exemplary embodiments, amplifier 40/400 may beany amplifier that is configured to (a) provide an amplitude modulation,or envelope power control function, (b) provide power gain whereapplicable, and/or (c) provide a feedback signal, i.e., a sample of theamplitude modulated (or power controlled) amplifier output power.

With reference now to FIGS. 1 & 2, in accordance with various aspects ofan exemplary embodiment of the present invention, RF envelope amplitudecontroller 60 may comprise a transmit waveform generator 100, loopfilter 300, and adaptive table waveform generator and update device(“ATWG”) 200. Furthermore, RF envelope amplitude controller 60 maycomprise a conversion component 500.

In accordance with various aspects of an exemplary embodiment of thepresent invention, and with reference now to FIGS. 2, 3, and 5,conversion component 500 is configured to receive a signal fromamplifier (40, 400), for example from feedback device 435 and convert itto a form of feedback that is useful in determining a correction to theenvelope control signal. In an exemplary embodiment, conversioncomponent 500 may transform and/or scale the signal from input 503, andoutput a signal, at output 509, that represents the envelope of thesampled RF output. The input signal of conversion component 500 may, forexample, be the RF output 440 reduced by the coupling factor of feedbackdevice 435.

In accordance with various aspects of an exemplary embodiment of thepresent invention, conversion component 500 is a log process andcomprises an input 503, output 509, and a logarithmic voltage detector506. In one exemplary embodiment, logarithmic voltage detector 506 isconfigured to realize an RF input voltage to output voltage transferfunction Vout=Vo1+K1*log(Vin/Vmin1) where Vin>=Vmin and K1, Vmin1 andVo1 are arbitrary scaling and offset terms inherent in the design of506. In another exemplary embodiment, logarithmic voltage detector 506is an integrated circuit of conventional design. For example, detector506 may be an AD8314, manufactured by Analog Devices, with an RF inputvoltage to output voltage transfer function Vout=Vo2+K2*log(Vin/Vmin2)where Vin>=Vmin and K2, Vmin2 and Vo2 are arbitrary scaling and offsetterms inherent in the design of the AD8314. Moreover, the output signalat conversion component output 509 may be a linear or nonlinear functionof, for example, the peak RF voltage at RF output 440. These othertransfer functions can include thermal detectors (that measure the powerof the sampled RF signal through its heating effect on the sensor),square law diode detectors, and/or the like.

Conversion component 500 may further comprise a voltage buffer 507 andassociated resistors 504 and 508. Buffer 507 is configured to provide alow impedance output at output port 509. Furthermore, other devices maybe used to provide a low impedance output at output port 509. Inaddition, conversion component 500 may be incorporated as part ofamplifier 400, as part of feedback device 435, as a separate component,etc.

In accordance with various aspects of an exemplary embodiment of thepresent invention, transmit waveform generator 100 is configured toreceive system controller signal(s), containing information such asmemory address, mode, and/or clock. The transmit waveform generator 100is also configured to generate a transmit waveform generator outputsignal(s) that facilitates adjusting the RF output signal of amplifier400 to within a close approximation of a desired RF amplifier outputenvelope.

In one exemplary embodiment, the conversion component 500 is alogarithmic voltage converter, and the transmit waveform generatoroutput signal is the logarithm of the desired RF output voltage. Thetransmit waveform generator output signal from transmit waveformgenerator output port 125 is summed with the feedback waveform signalfrom conversion component output 509. This sum is an error signal thatis one of two error signals that are the basis for the amplifier controlsignal received at amplifier control input 404. Transmit waveformgenerator 100 may be configured to provide this transmit waveformgenerator output signal to loop filter 300 and to adaptive tablewaveform generator 200. In one exemplary embodiment, the signal toadaptive table waveform generator 200 is inverted.

In accordance with various aspects of an exemplary embodiment of thepresent invention, transmit waveform generator 100 comprises digitalmemory 120, digital to analog converter (“DAC”) 115, and amplifiers 129and 131.

In accordance with one exemplary embodiment of the present invention,transmit waveform generator 100 is configured to receive address, mode,and/or clock digital inputs. These inputs may, for example, be receivedfrom a system controller 90. In an exemplary embodiment, transmitwaveform generator 100 is further configured to look up, in memory 120,a digital word corresponding to the address in the control signal andprovide the digital word to DAC 115. Transmit waveform generator 100 isfurther configured to provide the output of DAC 115, across resistor105, to operational amplifier 131, which is configured to provide a lowimpedance at transmit waveform generator output 125. Transmit waveformgenerator 100 may further comprise op amp 129 that is configured toinvert the output of DAC 115 and provide that output signal, at transmitwaveform generator output port 127, to ATWG 200.

Moreover, transmit waveform generator 100 may comprise any suitabledevice that is configured to receive system controller signals andgenerate a transmit waveform generator output signal(s) that facilitatesadjusting the RF output signal of amplifier 400 to within a closeapproximation of a desired RF amplifier output envelope.

In accordance with various aspects of an exemplary embodiment of thepresent invention, adaptive table waveform generator and update device(“ATWG”) 200 is configured to provide macro level adjustments to theamplifier control signal. In this regard, ATWG 200 may be configured tocompare the signal from transmit waveform generator 100 to the feedbacksignal and to create a second feedback signal that may be used tofurther adjust the amplifier control signal. In various embodiments,ATWG 200 only adjusts the amplifier control signal when the differencebetween the transmit waveform generator signal and the feedback signalexceeds a threshold.

In accordance with various aspects of an exemplary embodiment of thepresent invention, ATWG 200 is configured to adjust the amplifiercontrol signal using an adaptive table that stores the last value. Forexample, ATWG 200 may be configured to adjust an adaptive table when thedifference between the transmit waveform generator signal and thefeedback signal is large enough. The adjustment may consist ofincrementing or decrementing the value stored in the adaptive table. Inone exemplary embodiment, ATWG 200 is configured to read the valuestored in the adaptive table and to generate a signal based on the valuestored in the adaptive table. That signal may be provided by AWTG 200 toloop filter 300 to adjust the amplifier control signal (when it isappropriate that the amplifier control signal be adjusted).

With continued reference to FIG. 3, in accordance with various aspectsof an exemplary embodiment of the present invention, ATWG 200 comprisessystem controller (i.e., clock/control) input 290, transmit waveformgenerator input(s) 227, feedback input(s) 205, and ATWG output 235. Inaccordance with various aspects of an exemplary embodiment of thepresent invention, ATWG 200 further comprises adaptive table 210, DAC230, comparators 216 and 229, offset voltages, Vth, 236 and 239, andoperational clock timing delay 213. In one exemplary embodiment,adaptive table 210 consists of high speed random access memory that isconfigured to be sequentially addressed by the system clock insynchronization with reference waveform memory 120, so that address N of210 corresponds to address N of 120.

In one exemplary embodiment, the conversion component output signal atoutput 509 (input 205) and the reference waveform output signal attransmit waveform generator output 127 are connected to the inputs ofhigh-speed voltage comparators 216 and 229 through offset voltages 236and 239. In one exemplary embodiment, comparators 216 and 229 are LT1016comparators, manufactured by Linear Technology, Inc., and are capable ofnanosecond response times. An optional clock timing delay 213 isconfigured to define the comparator sampling time in relation to theclock edge. In one exemplary embodiment, ATWG 200 is configured so thatthe comparator outputs can update or modify the data contents at any oneadaptive table address N once during the duration of a transmit pulse,based upon the outputs of comparators 216 and 229 at sample N. Inanother exemplary embodiment, ATWG 200 is configured so that thecomparator outputs can update or modify the data contents at any oneadaptive table address (N-1) once during the duration of a transmitpulse, based upon the outputs of comparators 216 and 229 at sample N.Furthermore, ATWG 200 may be configured to modify the data contents ofthe adaptive table, based on the comparator outputs, in any othersuitable way.

By way of example, if the voltage of the feedback signal from logprocessor output 509 exactly matched the reference waveform voltage fromoutput 127, no system correction would be made. In this case, the offsetvoltage devices 236 and 239 are configured to maintain the outputs ofcomparators 216 and 229 respectively at ‘low’ levels (most negative)despite small offsets and noise at the comparator inputs, and no changeswould be made to the data in the adaptive table.

Continuing this example, if the detected log output voltage at output509 (input 205) is less positive than the reference waveform voltage atinputs 227 by more than the voltage of offset voltage 236, thencomparator 216 goes ‘high’ (most positive). This ‘high’ state ofcomparator 216 decrements (makes less positive) the value of theadaptive table data at the current address.

Similarly, if the detected log output voltage at output 509 (input 205)is more positive than the reference waveform voltage at input 227 bymore than the voltage of offset voltage 239, then comparator 229 goes‘high.’ This ‘high’ state of comparator 229 increments (makes morepositive) the value of the adaptive table data at the current address.

The value of the integer by which the adaptive table is incremented ordecremented is not critical. However, excessively large values wouldconstitute a large fraction of the maximum data word size in theadaptive table, thereby reducing the accuracy of the ‘A’ loop correctionprocess.

In one exemplary embodiment, the sense or polarity of DAC 230 isconfigured so that the most positive data value corresponds to maximumpositive output at ATWG output port 235. Furthermore, the transmittersystem may be configured such that ATWG output port 235 is in electricalcommunication with the inverting input of loop filter 300, therebycreating a negative feedback loop comprising blocks 200, 300, 400, and500. The offset voltage devices 236 and 239 are further configured tolimit the corrective action of the ‘A’ adaptive table loop to thoseregions of the waveform exhibiting comparatively large errors. Asillustrated in FIGS. 6 & 7, the contents of the adaptive table continueto change as long as the threshold of either comparator is exceeded, andremain uncharged thereafter unless changes in operating conditions suchas temperature, output power selection, or waveform selection occur.

Although one exemplary embodiment is described here, the transmittersystem may also comprise other systems that comprise a first feedbackloop for correcting an RF amplifier output signal envelope to areference envelope, and a second feedback loop, wherein the secondfeedback loop provides additional correction when the error between areference envelope and the actual envelope is large.

In accordance with various aspects of an exemplary embodiment of thepresent invention loop filter 300 is configured to combine signalsrepresenting a reference waveform signal, a first feedback signal, and asecond feedback signal and to generate an amplifier control signal thatfacilitates adjusting the output signal from the RF amplifier to meetthe accuracy requirements of the desired reference waveforms.

In one exemplary embodiment of the present invention, loop filter 300comprises initial amplifier control signal input 325, first feedbackinput 305, second feedback input 335, and output 304. In this exemplaryembodiment, loop filter 300 also comprises an operational amplifier 323,resistors 307, 320, 347, 326, 333, and 331, capacitors 336, 340 and 341,diode 345, and ground 350. Operational amplifier 323, in thisembodiment, is connected as an inverting summer, driven by voltagesources from inputs 305, 335, and 325. When all functional blocks areinterconnected as in FIG. 3, loop filter 300, with the exception ofdiode 345, comprises a conventional error amplifier (i.e., a ‘loopfilter’) having a suitable phase lead and lag compensation configured toprovide closed loop stability. Diode 345 is configured to limit theoutput voltage of operational amplifier 323 to small negative valuesduring transient conditions.

Nevertheless, other devices may be used to generate an amplifier controlsignal based a reference waveform signal, an analog feedback signal, anda predictive approximation feedback signal, wherein the amplifiercontrol signal is used to adjust the output signal from the RF amplifierto meet the accuracy requirements of the desired reference waveforms.

Furthermore, other components and assemblies may be used, and RFenvelope amplitude controller 60 may comprise any device(s) that areconfigured to exhibit transfer functions equivalent to that describedabove, as measured individually between the inputs 325, 290, 227, or 503and the output 304.

In accordance with various aspects of an exemplary embodiment of thepresent invention, the amplified RF output signal is provided to anantenna. Furthermore, the RF output signal may pass through other lineardevices prior to reaching the antenna.

In one exemplary embodiment of the present invention, and with referenceto FIG. 8, an exemplary RF envelope amplitude control method 800comprises the steps of: providing a reference waveform signal to atransmitter RF amplifier (step 810), receiving a feedback signal basedon the RF output of the RF transmit amplifier (step 820), comparing thefeedback signal to the original reference waveform signal (step 830),adjusting an adaptive table (step 833), providing an adjustment to thereference waveform signal based directly on the feedback signal andbased on the comparison (step 840), and/or providing an adjustedreference waveform to the transmitter RF amplifier (step 850).

Providing an adjustment to the reference waveform signal (step 840) mayfurther comprise the steps of combining the reference waveform signalwith the feedback signal and with a second feedback signal, which secondfeedback signal represents the difference between said referencewaveform signal and said feedback signal. The second feedback signaladjusts the reference waveform signal such that the result is a closeapproximation of the amplifier control signal that will cause theamplifier to generate an RF output within a close approximation of aspecified waveform envelope. In contrast, the first feedback signalmakes fine adjustments to the amplifier control signal to further reduceor minimize the RF output signal's deviation from the specified waveformenvelope.

The invention may be described herein in terms of functional blockcomponents, optional selections and/or various processing steps. Itshould be appreciated that such functional blocks may be realized by anynumber of hardware and/or software components suitably configured toperform the specified functions. For example, the invention may employvarious integrated circuit components, e.g., memory elements, processingelements, logic elements, look-up tables, and/or the like, which maycarry out a variety of functions under the control of one or moremicroprocessors or other control devices. Similarly, the softwareelements of the invention may be implemented with any programming orscripting language such as C, C++, Java, COBOL, assembler, PERL, VisualBasic, SQL Stored Procedures, extensible markup language (XML), with thevarious algorithms being implemented with any combination of datastructures, objects, processes, routines or other programming elements.Further, it should be noted that the invention may employ any number ofconventional techniques for data transmission, messaging, dataprocessing, network control, and/or the like.

It should be appreciated that the particular implementations shown anddescribed herein are illustrative of the invention and its best mode andare not intended to otherwise limit the scope of the invention in anyway. Indeed, for the sake of brevity, conventional amplifiers andelectronic components, application development and other functionalaspects of the systems (and components of the individual operatingcomponents of the systems) may not be described in detail herein. Itshould be noted that many alternative or additional functionalrelationships or physical connections might be present in a practical RFenvelope amplitude control system.

As may be appreciated by one of ordinary skill in the art, the inventionmay take the form of an entirely software embodiment, an entirelyhardware embodiment, or an embodiment combining aspects of both softwareand hardware or other physical devices. Furthermore, the invention maytake the form of a computer program product on a computer-readablestorage medium having computer-readable program code means embodied inthe storage medium. Any suitable computer-readable storage medium may beutilized, including hard disks, CD-ROM, optical storage devices,magnetic storage devices, and/or the like.

These computer program instructions may also be stored in acomputer-readable memory that may direct a computer or otherprogrammable data processing apparatus to function in a particularmanner. The computer program instructions may also be loaded onto acomputer or other programmable data processing apparatus to cause aseries of operational steps to be performed on the computer or otherprogrammable apparatus to produce a computer-implemented process suchthat the instructions which execute on the computer or otherprogrammable apparatus include steps for implementing the functionsspecified in the flowchart block or blocks.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, it may be appreciated thatvarious modifications and changes may be made without departing from thescope of the invention. The specification and figures are to be regardedin an illustrative manner, rather than a restrictive one, and all suchmodifications are intended to be included within the scope of invention.Accordingly, the scope of the invention should be determined by theappended claims and their legal equivalents, rather than by the examplesgiven above. For example, the steps recited in any of the method orprocess claims may be executed in any order and are not limited to theorder presented.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as critical, required, or essentialfeatures or elements of any or all the claims. As used herein, the terms“comprises”, “comprising”, or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus. Further, noelement described herein is required for the practice of the inventionunless expressly described as “essential” or “critical”.

1. An RF envelope amplitude control system, the system comprising: an RFamplifier configured to receive an RF input signal and to output an RFoutput signal, wherein said RF amplifier is associated with a feedbackdevice that is configured to create a first feedback signal representingthe power in said RF output signal; a transmit waveform generatorconfigured to generate a reference waveform signal; an adaptive tablewaveform generator configured to compare said reference waveform signaland said first feedback signal and to create a second feedback signalbased on the distortion between said reference waveform signal and saidfirst feedback signal; and a loop filter configured to combine saidreference waveform signal, said first feedback signal, and said secondfeedback signal to form an amplifier control signal, wherein saidamplifier control signal is provided to said RF amplifier to adjust saidRF output signal to reduce nonlinearity in said RF output signal withreference to a specified RF envelope.
 2. The RF envelope amplitudecontrol system of claim 1, further comprising a conversion componentconfigured to convert said first feedback signal to a voltage signalthat is representative of the RF envelope but is not necessarily alinear function of the first feedback signal.
 3. The RF envelopeamplitude control system of claim 2, wherein said conversion componentfurther comprises a logarithmic detector, and wherein said feedbackdevice is a passive RF directional coupler.
 4. The RF envelope amplitudecontrol system of claim 1, wherein said amplifier control signal isprovided to said RF amplifier to minimize the distortion between said RFoutput signal and a specified RF envelope.
 5. The RF envelope amplitudecontrol system of claim 1, further comprising a memory, wherein saidtransmit waveform generator is configured to receive a predefined RFenvelope waveform, and to output said predefined RF envelope waveform assaid reference waveform signal.
 6. The RF envelope amplitude controlsystem of claim 1, wherein said adaptive table stores a value for thewaveform in a table and wherein said value is adjusted based on saidcomparison between said first feedback signal and said referencewaveform.
 7. The RF envelope amplitude control system of claim 1,wherein said specified RF envelope is any envelope associated with anywaveform for communication or navigation that utilizes RF or opticalpulses.
 8. The RF envelope amplitude control system of claim 1, whereinsaid specified RF envelope is that associated with one of the followingtransmission modes: TACAN, TDMA, FSK, GMSK, MSK, and PSK.
 9. An RFenvelope amplitude control system, the system comprising: an RFamplifier configured to receive an RF input signal and to output an RFoutput signal, wherein said RF amplifier is associated with a feedbackdevice that is configured to create a first feedback signal representingthe power in said RF output signal; a transmit waveform generatorconfigured to generate a reference waveform signal; an adaptive tablewaveform generator configured to compare said reference waveform signaland said first feedback signal and to create a second feedback signalbased on the difference between said reference waveform signal and saidfirst feedback signal; and a loop filter configured to form an amplifiercontrol signal, wherein said amplifier control signal is based on saidreference waveform signal, said first feedback signal, and said secondfeedback signal, wherein said second feedback signal facilitates apredictive and approximate correction to said amplifier control signal,and wherein said first feedback signal facilitates reducing thedifference between said RF output signal and a specified RF envelope,and wherein said amplifier control signal is provided to said RFamplifier to adjust said RF output signal to reduce the differencebetween said RF output signal and a specified RF envelope.
 10. The RFenvelope amplitude control system of claim 5, wherein said predefined RFenvelope waveform is stored at an address of said memory, and saidaddress is received at said transmit waveform generator.
 11. The RFenvelope amplitude control system of claim 1, wherein said secondfeedback signal facilitates a pre-distortion correction to saidamplifier control signal, and wherein said first feedback signalfacilitates reducing the nonlinearity of said RF output signal incomparison to said specified RF envelope.
 12. The RF envelope amplitudecontrol system of claim 1, wherein said amplifier control signal isprovided to said RF amplifier to adjust said RF output signal to reducethe distortion between said RF output signal and said specified RFenvelope.